The present invention relates generally to photoemitting devices and, more particularly, to a photomultiplier having a photocathode with a photoemitting surface characterized by a plurality of acicular or large aspect-ratio photoemitting structures, and a method for the production thereof.
Photomultiplier tubes, as illustrated in FIG. 1, operate by making use of the process of photoemission, whereby a photon of sufficient energy to actuate the emission of an electron is captured by a photocathode 12. The resultingly emitted photoelectron exits the photocathode 12; while the initial emission direction is unpredictable, the photomultiplier tube 10 is designed such that the majority of photoelectrons emitted from the photocathode 12 are directed towards an amplifier 14. The amplifier 14 is typically a series or string of dynodes 16 ending in a terminal anode 18 positioned to maximize the probability of reception of photoelectrons emitted from the photocathode 12. The anode 18 generates an electric potential to help direct the flow of photoelectrons toward the anode 18. A photoelectron striking a dynode 16 in the amplifier string actuates the secondary emission of additional electrons. An electron stream is thusly created and urged toward the anode 18, with each dynode 16 sequentially receiving a number of electrons travelling from the previous dynode 16 in the string and increasing the electron flow by releasing a plurality of secondary electrons to the next dynode 16 for each one received from the previous dynode 16. In this way, a photomultiplier tube 10 can amplify the single-electron signal generated by the emission of one photoelectron actuated by the reception of a single photon by the photocathode 12 on the order of a million (1,000,000X) or more. Although the above describes a typical arrangement, alternative arrangements for multiplication are commonly used. One such alternative arrangement is that of the multichannel plate in which multiplication occurs in a plurality of high-aspect ratio channels, which have been formed into a material suitable for such multiplication.
FIG. 2 illustrates a typical prior art photocathode 12 in greater detail. The photocathode 12 is formed as a flat layer of photoemissive material. In this embodiment, the photoemissive material is formed on a substrate 20 (here, a photomultiplier tube window, although the substrate may be separate from the window). The photocathode 12 has two major sides, a first major side 22 through which incident photons, in the energy range corresponding to ultraviolet, visible, or near infrared radiation, hereinafter referred to as optical or light, are received and a second, opposite major side 24 through which photoelectrons are desired to be emitted. The second major side 24 is substantially smooth and positioned equidistant from the first major side 22. While the photocathode 12 of FIG. 2 is depicted as being substantially flat, it should be noted that some photocathodes 12 have some degree of curvature.
In a second common implementation, referred to as reflection mode, the photocathode is substantially thick and the optical photons and the photoelectrons are emitted from the same surface. Following emission, the photoelectrons continue on to the dynode string or equivalent amplifying system as described above. In this geometry, the photocathode is effectively infinitely thick so that photons can interact and photoelectrons can originate from a point much further from the photocathode than is the case for optically thin photocathodes as described above.
One important parameter characterizing the efficiency of a photomultiplier is quantum efficiency. The quantum efficiency of the photomultiplier tube is defined as the ratio of the number of output pulses to the number of input photons. Typically, photomultiplier tubes have a maximum quantum efficiency of about 25%. Two major competing factors currently combine to limit the quantum efficiency of typical photomultiplier tubes. First, a photoelectron emitted from a typical photocathode may be recaptured by the photocathode. Even if the photoelectron is not recaptured, it may leave the photocathode travelling in any direction, including towards the photon source and away from the amplifier. Second, photons may pass through the photocathode without being captured. If the photocathode is made thicker so as to increase the incidence of photon absorption, it becomes more difficult for a photoelectron to escape the photocathode. Conversely, if the photocathode is made thin enough so that substantially all of the photoelectrons can escape the photocathode, the photocathode will also allow an unacceptably large number of photons to pass therethrough without generating photoelectrons. Photocathodes operating in the conventional geometry and photocathodes operating in the reflection geometry have approximately the same quantum efficiency indicating that the conventional geometry is well optimized for balancing these two phenomena. Currently, the photocathode is produced with an intermediate compromise thickness that correlates to a maximum quantum efficiency of about 20-25% although very recently GaAs and GaAlAs photomultiplier tubes have been commercially introduced with quantum efficiencies, at the most efficient wavelengths, and when cooled, of up to 45%.
There is therefore a need for a photocathode design that allows for increased photoelectron emission without correspondingly increased photon transmission and, more preferably, with increased photon absorption resulting in photoemission. The present invention is directed towards meeting this need.
The present invention relates to a photocathode design having an increased surface-to-volume ratio for the purpose of increased quantum efficiency. The photocathode includes a transparent substrate, upon one major side of which is formed a plurality of large aspect-ratio structures, such as needles, cones, or pyramids. In a degenerate case, the structure can be as simple as a single large aspect-ratio structure, such as a film, arranged such that, effectively, photoelectrons may be emitted from both sides of the structure. The large aspect-ratio structures are at least partially composed of a photoelectron emitting material, i.e., a material that emits a photoelectron upon absorption of an optical photon. The large aspect-ratio structures may be substantially composed of the photoelectron emitting material (i.e., formed as such upon the surface of a relatively flat substrate) or be only partially composed of a photoelectron emitting material (i.e., the photoelectron emitting material is coated over large aspect-ratio structures formed from the substrate material itself.) The large aspect-ratio nature of the photocathode surface allows for an effective increase in the thickness of the photocathode relative the absorption of optical photons, thereby increasing the absorption efficiency of incident photons, without substantially decreasing the effective thickness of the photocathode relative the escape incidence of the photoelectrons. This is at least in part because the large aspect-ratio nature of the photocathode electron emission surface facilitates the emission of electrons. In other words, it is much easier for an electron to be emitted from the tip of a cone or needle than from a flat surface.
One object of the present invention is to provide an improved photocathode device. Related objects and advantages of the present invention will be apparent from the following description.